1. Field of the Invention
The present invention relates to a lithographic apparatus and method for determining a measure of Z position errors/variations and/or a measure of substrate table (or “chuck”) flatness.
2. Description of the Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In this case, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern corresponding to an individual layer of the IC. This is done using a projection system that is between the reticle and the substrate and is provided to image an irradiated portion of the reticle onto a target portion of a substrate. The projection system includes components to direct, shape and/or control a beam of radiation. The pattern can be imaged onto the target portion (e.g. including part of one, or several, dies) on a substrate, for example a silicon wafer, that has a layer of radiation-sensitive material, such as resist. In general, a single substrate contains a network of adjacent target portions that are successively exposed. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through the projection beam in a given direction, usually referred to as the “scanning” direction, while synchronously scanning the substrate parallel or anti-parallel to this direction.
In such a lithographic apparatus, an interferometer might be used for measuring displacements with a high accuracy. U.S. Pat. No. 4,784,490, the contents of which are incorporated herein by reference, discloses a typical high stability plane mirror interferometer. The interferometer (also sometimes called interferometer system) comprises a polarizing beam splitter for splitting a beam into a measurement beam and a reference beam, a reference mirror and a measurement mirror for reflecting the reference beam and the measurement beam respectively, and a phase detector for detecting the beam resulting from interference between the measurement beam and the reference beam. By displacement of the measurement mirror, a total length of a measurement path that is travelled by the measurement beam is altered, which results in a phase shift at the phase detector. Using the detected phase shift, it is possible to determine the displacement of the mirror.
Although interferometer systems can provide highly accurate measurements, a problem is that measurement errors occur due to unflatness of the measurement mirror. When the measurement mirror is displaced in a direction perpendicular to the measurement beam, the measurement beam will touch the mirror at a different area on the mirror. Unflatness of the mirror thus results in an error in the distance measurement by the interferometer.
Mirror unflatness is a particular problem when an interferometer system is used in combination with an array of level sensors in order to determine both the x and z positions of the substrate table and the height of the wafer that is on it. In this case, measuring the x and z positions is typically done using appropriately positioned X and Z mirrors. Measuring the level of the wafer surface is done using the level sensors. Using the Z-mirror position and the wafer level, it is possible to determine an absolute measure of the wafer height, on the assumption that the wafer height can be expressed as the difference between the vertical position of the wafer surface and the vertical position of the substrate table, i.e. wafer height=(vertical position of wafer surface-vertical position of substrate). An arrangement of this type is described in more detail in U.S. Pat. No. 6,674,510, the contents of which are fully incorporated herein by reference.
Problems with this arrangement can arise because when two level sensors positioned at different x-positions (such as an optical level sensor and an air gauge) are used to measure a single wafer point the wafer table has to be moved so that wafer point is moved from a position at which it can be measured by the first sensor to a position at which it can be measured by the second sensor. This means that different substrate table x-positions are applied. Hence, different positioning errors result for the two measurements. Because the wafer itself may not be flat, differences in the x-position may mean that the second level sensor measurement is taken at an x-position that has a different level to that of the previous measurement. This is turn means that the measurement radiation will be incident on the Z-mirror at a different position. Because the Z-mirror may not be perfectly flat, the wafer height (which is dependent on the Z-mirror vertical position) may be incorrectly calculated. Hence, in order to compare the results of the measurements an absolute Z-mirror map on the measurement side is required to correct for mirror errors.